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Ferromagnetic-semiconductor hybrid nanostructures for Si-based spintronics: Synthesis and properties

Final Activity Report Summary - SIMAG (Ferromagnetic-semiconductor hybrid nanostructures for Si-based spintronics: Synthesis and properties)

The goal of spintronics, or spin electronics, is to find ways to use the spin degree of freedom in solid-state systems as charge degree of freedom is used in electronic devices. Spintronics research initially focussed on metal-based structures, leading to advanced sensors and memories. Nowadays, semiconductor-based spintronic concepts are attracting much attention, since semiconductor-based structures offer opportunities that are inaccessible to metal-based structures. Moreover, the use of semiconducting magnetic elements may enable to reduce the large currents that are now required in metal-based spintronic devices. In order to utilise the spin degree of freedom in semiconductors on which the current electronics is based, we need to fabricate appropriate materials, understand the spin-dependent phenomena and control the spins.

This project focussed on the synthesis, using Molecular beam epitaxy (MBE), and the analysis of the properties of hybrid ferromagnet semiconductor structures for spintronics. The hybrid structures could be divided in three categories:
1. single-phase Diluted magnetic semiconductors (DMS);
2. granular materials consisting of ferromagnetic nanoclusters embedded in a semiconducting matrix; and
3. ferromagnetic metal or semiconductor thin-film heterostructures with extended planar interfaces.

We studied the crystallographic structure of granular gallium and manganese arsenide (GaAs:MnAs), consisting of magnetic MnAs nanoclusters embedded in semiconducting GaAs. This material could be synthesised by annealing MBE-grown diluted (Ga,Mn)As. We found that the constrained MnAs clusters held remarkably large and anisotropic strain at room temperature, as mentioned in the Applied Physics Letters 86, 161903, in 2005. The cluster strain resulted in increased Mn-As-Mn distance and enhanced Curie temperature, as compared to bulk MnAs. We used the continuum elasticity theory to calculate the thermal variation of the MnAs-cluster lattice parameters, as published in Physical Review B 72, 115206, in 2005.

We concluded that the strained state of the MnAs clusters made the coarsening process sensitive to anisotropies and inhomogeneities in the stress field of the matrix during thermal annealing. We therefore proposed to advantageously use this property to guide the orientation and lateral assembly of the MnAs precipitates within the GaAs matrix, as analysed in the Journal of Vacuum Science and Technology B 23, 1700 in 2005, which could lead to enhanced magnetic and magnetotransport properties. Future work was planned to focus on Si-based diluted magnetic semiconductors and half-metallic Heusler alloys intended to be integrated, as thin films, in the Si platform.